This invention relates generally to nitriding of steel articles, and more particularly to a steel article which is to be mated with a separate article so as to make a sliding or rolling face-to-face contact and an improved nitriding method for the production of the same.
Hardening of a surface region of steel articles can be achieved by diffusing nitrogen into the surface region at elevated temperature. This process is familiar under the term of nitriding. In principle, nitriding is a strain hardening of steel attributable to the formation of stable compounds (nitrides). The usual source of active nitrogen for nitriding is ammonia gas. At the nitriding temperature, at least part of the ammonia gas decomposes at the surface of the steel, liberating active nitrogen. The liberated nitrogen permeates and diffuses into the steel, so that the steel is gradually nitrided from the surface.
When nitriding is effected on a steel article which is to be mated with a separate article in a manner to make a sliding or rolling face-to-face contact, there is a problem that the nitrided article has rather a poor property of smoothly fitting the mated article, particularly during an initial stage of use, whether the mating takes the form of a rubbing contact as in the case of bushings or a meshing contact as in gears.